xet upload: avoid downloading blobs twice?

[coyotte508]

Follow up #1718

Currently, blobs are processed twice, once for sha256 hashing and once for actual upload (creating xorbs, ...)

This works fine for local files, but when editing or uploading a remote file, downloading the file twice sucks.

Separating the sha step from the xorb/shard step is good though, we don't want to write useless data if the sha already exists in the repo.

IMO a solution would be using https://developer.mozilla.org/en-US/docs/Web/API/IndexedDB_API or https://developer.mozilla.org/en-US/docs/Web/API/Cache - to store the downloaded file on the disk temporarily.